This invention relates to stainless steel with a passive state fluorinated film formed thereon and a device using the same.
Various kinds of special gases are used in semiconductor production processes, and most of them exhibit corrosive properties themselves or they react with moisture in the atmosphere and are formed into substances exhibiting corrosiveness. Usually, chambers for handling these special gases, piping therefor, valves essential for supplying the gases, or the like are made of stainless steel, and have such a feature as they are easily corroded by the above special gases.
Moreover, semiconductor devices have increasingly improved integration year by year, and the dimensions of unit elements have also been decrease year by year as the integration is improved. As the unit element is decreased in dimension, dimensions of a pattern to be drawn on a silicon substrate in the semiconductor manufacturing process have become extremely small, and it has become difficult to draw required lines with an aligner using g-line and i-line which have conventionally been used.
For this reason, an excimer-laser aligner is worthy of use at present. Dilute fluorine gas is filled in a laser chamber of an excimer laser aligner.
FIG. 5 shows an outline drawing of the laser chamber.
As shown in FIG. 5, the laser chamber 1 comprises two stainless housing members 13, 14, and the housing members 13. 14 are sealed with an O-ring 15. A cathode 18 is fixed to the housing member via an insulating body 16 and a cathode supporting member 17. An anode 19 is fixed to the housing 13 via an anode supporting member 20. 21 is a connector for connecting the cathode 18 with a pulse oscillator (not shown in the figure).
Moreover, 22 is a sealing member such as a O-ring.
In the laser chamber 1, a blower for circulating a gas, a heat exchanger, and other devices (not shown) are arranged.
27 is a pipe arrangement for fluorine gas supply line made of stainless steel.
However, in such a device, the inner wall of the pipe arrangement 27 for the dilute fluorine gas supply line made of stainless steel, the inner walls of the housings 13, 14, or the outer walls of the devices arranged in the chamber 1 reacts with the fluorine gas, and the fluorine gas is consumed. As the result, it is difficult to stably supply the dilute fluorine gas. Therefore, in the present situation, the gas piping 27 for the dilute fluorine gas supply line has been made up first, and then the dilute fluorine gas has been filled in the supply line, to let the fluorine gas react with the inner wall of the supply line before use.
However, it is impossible to operate a semiconductor production line in the pre-reaction period. Moreover, even in the pre-reacted supply line, the reaction proceeds further between the fluorine gas and the inner wall of the supply line during use, as a result, it is difficult at the present to use the dilute fluorine gas stabilized in density or the like.
In order to solve this problem, the applicants of the present invention have repeatedly researched corrosivity and reactivity of metal surfaces, and as a result, they found out that it was possible to form a passive state fluorinated film having good corrosion resistance against corrosive gasses, above all, fluorine gas by making fluorine gas positively react with a stainless steel surface for fluorination at a sufficient temperature, to form thereon a film of a metal fluoride as a main component, and then heat-treating this film in an atmosphere of an inert gas, and they already applied for the patents based on these invention (refer to the Japanese Patent Provisional Publications 2-263972 (263972/1990), 3-213656 (213656/1991), and 5-315 (3315/1.9.93A).
As the result of their continuing research for new technology, the applicants of these inventions have found out the following fact.
Namely, any of the passive state fluorinated films applied for the above patents are not less than 500 xc3x85 in thickness. When the passive state fluorinated films have such a thick film thickness, firstly, metal deposits are produced at the time of welding the gas piping or the like and the produced metal deposits stay in the piping as particles. Secondly, when a thick passive state fluorinated film is formed on a sealing surface of a joint and a valve seat face, it causes leakage disabling them to be usable.
As countermeasures, it can be considered that the passive state fluorinated film be removed from the welding part before welding, and that the passive state fluorinated film be formed by masking only the sealing surface of the joint and the surface or the valve seat.
However, both methods have lacked productivity and have been difficult for practical use. Above all, it has taken a great amount of time to complete the piping arrangement because it has to be welded at as many as hundreds of points.
The purposes of this invention are to provide stainless steel coated with the passive state fluorinated film thereon, which is easy to construct and does not produce particles even though it is welded, and to provide a device for using the same.
The purposes of this invention are also to provide stainless steel coated with the passive state fluorinated film thereon which does not cause leakage even if the passive state fluorinated film is formed on the sealing surface of the joint and the seat surface of the valve, and to provide a device using the same.
The stainless steel coated with the passive state fluorinated film in accordance with this invention is characterized in that at least a part of the surface is coated with a passive state fluorinated film of 190 xc3x85 or less in thickness consisting of metal fluoride as a main component.
Here, as stainless steel, austenitic stainless steel, ferritic stainless steel, and other stainless steel can be used. Especially, SUS316 or SUZS316L is preferred for the use.
According to this invention, a passive state fluorinated film is formed on the stainless steel, and the thickness of the passive state fluorinated film is made to be 190 xc3x85 or less. The particles produced at the time of welding decrease sharply under 190 xc3x85 as a critical value, and sealing performance is also radically improved.
As for a lower limit, 5 xc3x85 are preferred. The reason is that a film thinner than this is difficult to exist as a film.
The formation of the passive state fluoride is preferred to be performed, for example, in the following method.
A stainless steel surface is mirror-finished by electrolytic polishing or the like, and then it is baked in a high-purity inert gas (for example, nitrogen, argon, helium, etc.) to remove moisture content from the stainless steel surface.
For an inert gas, it is preferred to reduce a density of impurities such as moisture to 50 ppb or less, and a density of 20 ppb or less is more preferable, and 15 ppb or less is most preferable.
Following the above, a film of a metal fluoride is formed, at least, on a part or the whole of the stainless steel surface (fluorination).
For a gas at the fluorination, 100% fluorine gas or dilute fluorine gas with an inert gas is used. Then, for the gas at the fluorination, it is favorable to use a gas with a 50 ppb or less impurity content such as moisture, and a gas with a 20 ppb or less impurity content is more favorable, and a gas with a 15 ppb or less impurity content is further more favorable.
Temperatures for the fluorination are preferred to be 50xc2x0 C. or higher, and temperatures of 100 to 200xc2x0 C. are more preferable, and temperatures between 120 to 170xc2x0 C. are most preferable. A fluorination time is preferred to be not shorter than 10 minutes, and the fluorination time for 30 minutes to 5 hours is more preferable.
The thickness of the passive state fluorinated film can be controlled by varying the temperature, time, and density of fluorine in the inert gas at the time of fluorination as necessary.
For example, when the fluorination is performed for three hours at 150xc2x0 C. by using 1% fluorine gas diluted with nitrogen gas, a 190 xc3x85 thick passive state fluorinated film is formed. Heat treatment at 50xc2x0 C. or higher after the fluorination is preferable, and it is more preferable to conduct the treatment at 100 to 400xc2x0 C. A treatment time for 10 minutes or longer is preferable, and that for 30 minutes to 15 hours is more preferable.